1. Field of the Invention
The present invention relates to a cleaning blade to be used in a copying machine for forming images by electrophotography, and to a cleaning apparatus, a process cartridge, and an image forming apparatus using the same.
2. Description of the Related Art
Generally, formation of an image by electrophotography or an electrostatic recording method is carried out by forming an electrostatic latent image on an image holding member such as a photoreceptor, forming a toner image by developing the electrostatic latent image with a developer, and successively transferring and fixing the toner image on a recording medium. An image forming apparatus employing the above-mentioned image forming method is generally provided with a cleaning apparatus for cleaning the toner remaining on the image holding member after the transfer of the toner image.
Various types of cleaning apparatuses are known and, typically, a blade cleaning method in which a cleaning blade (a contact plate having elasticity is generally used) is installed so as to have a contact with the image holding member and thus scrape off any toner remaining on the image holding member has been employed in many cases.
In the case of low-to-middle speed apparatuses, contact type charging apparatuses such as charging rolls have widely been used as the charging members of an image holding member. The reasons for the wide use of a contact type charging apparatus are that as compared with a non-contact type charging apparatus using Corotron charging, the contact type apparatus is accompanied with very little ozone generation and is therefore environment-friendly, and that the contact type apparatus is compact and economical in cost since it requires no ozone filter or air blow.
On the other hand, a contact type charging apparatus has the problem that the deposition amount of electrically discharged residue to a photoreceptor is considerably larger than that of a non-contact type charging apparatus using corotron charging. This is because the electric discharge region is very close to the photoreceptor, although the absolute amount of the electrically discharged residue is less than that generated in the case of a non-contact type charging apparatus. Therefore, in the case of using a contact type charging apparatus and a cleaning blade in combination, there occur problems of wearing, cracking, and curling of the cleaning blade and torque increase of the photoreceptor operation system owing to the increase of the friction coefficient.
To prevent these problems, apparatuses capable of removing the electrically discharged residue discharged onto the surface of an image holding member and lowering the friction force on a cleaning blade have been proposed For example, methods for removing the electrically discharged residue by sliding a magnetic brush and a sponge roll containing an abrasive agent on the surface of an image holding member have been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 10-143039 and 1-090479). Further, there have been other methods proposed which provide improved wear resistance of a cleaning blade by selecting proper materials such as, for example, a method of using a material having low friction and high hardness for a portion of a cleaning blade which is brought into contact with an image holding member (see, for example, JP-A 2001-343874) and a method of using a material having high modulus (high hardness) for a portion of a cleaning blade which is brought into contact with an image holding member (see, for example, JP-A 2003-241599).
In the case where a two-component development method using a developer containing a magnetic carrier and a toner is employed, the toner development is carried out by applying predetermined development bias to a development roll and thereby applying a development electric field between the development roll and an image holding member for transferring the toner to an electrostatic latent image and in such a process, there occurs the BCO (Bead Carry Over) phenomenon in which a portion of the magnetic carrier is transferred to the surface of the image holding member owing to the electrostatic attraction force.
On the other hand, unlike normal carrier of a substantially spherical shape, fine powder carrier pulverized carrier) generated in a developer production process or a development apparatus often has a sharp debris shape. Accordingly, if BCO takes place, at the time of transferring a toner image to an toner-receiving body, the fine powder carrier is easily buried in the surface of the image holding member owing to the transfer electric field or the transferring pressure between the image holding member and the toner-receiving body and the fine powder carrier is firmly fixed in the surface of the image holding member once adhered to the surface.
Along with a rotation of the image holding member, the carrier thus adhered to the surface of the image holding member is repeatedly brought into contact with a cleaning blade being brought into contact with the surface of the image holding member, such that edge-cracking takes place in the cleaning blade. To prevent such a phenomenon, apparatuses for trapping the carrier adhered to the surface of the image holding member have been proposed. For example, an apparatus for trapping the carrier by forming an electric field between a trapping member and a photoreceptor (see, JP-A No. 62-262074) and apparatuses for trapping the carrier by magnetic attraction force (see, JP-A No. 3-120577 and Japanese Utility Model Application Publication (JP-Y) No. 53-32599) have been proposed.
However, neither addition of means for removing the electrically discharged residue in order to improve the wear resistance of the cleaning blade nor addition of carrier trapping means for trapping the fine powder carrier which adheres to the surface of the image holding member along with occurrence of BCO and becomes a cause of edge-cracking, to an image forming apparatus is desirable in terms of miniaturization and cost cutting. Further, carrier with a sharp debris shape being transferred to and buried in the surface of the image holding member at the time of transferring a toner image needs to be prevented. In this case, between the development step and the transfer step, carrier with a sharp debris shape adhered to the surface of the image holding member has to be removed without disturbing the loner image developed on the image holding member.
Therefore, to trap the carrier with a small particle diameter and a sharp debris shape adhered to the surface of the image holding member, a magnetic field or electric field more intense than that in the case of trapping a carrier with a normal particle diameter is required To generate a more intense magnetic field or electric field, it is necessary to set the trapping means closer to the photoreceptor. However, the trapped carrier scratches the toner image on the image holding member during transportation if the trapping means is simply set closer to the photoreceptor and thus it is very difficult to completely remove the carrier shifted to the image holding member without disturbing of the toner image.
On the other hand, if a material with high hardness or high modulus is employed as the material for a portion of the cleaning blade to be brought into contact with the image holding member to improve the wear resistance of the cleaning blade, in general, the resistance is improved but the elasticity is decreased. If the elasticity is decreased, rubber-like properties are lost and it becomes difficult for the cleaning blade to expand. Owing to difficulty of expansion, when foreign substances such as carrier debris buried in the surface of the image holding member along with the occurrence of BCO pass the contact part between the edge of the cleaning blade and the surface of the image holding member, the edge tip end cannot be deformed in response the force of the foreign substances deforming the edge and is easily cracked.
Accordingly, to prevent the edge-cracking of the cleaning blade, it is advantageous that a material having a somewhat low hardness so that the edge tip end deforms (expands) when foreign substances pass the contact part between the edge of the cleaning blade and the surface of the image holding member is used to form the edge tip end part. However, a material with low hardness is inferior in wear resistance, so that it cannot maintain good cleaning capability for a long period of time.